Liquid crystal compounds are an important part of the liquid crystal display devices. Liquid crystal displays have many outstanding advantages in, for example, that they can be made to exhibit thin-thickness and light-weight, require low driving voltage and low power consumption. Also, they are non-emissive and are easy on the eyes. Because of these advantages, liquid crystal displays are widely considered as the mainstream display devices of the next generation.
Most of the conventional liquid crystal display compounds are of the nematic phase, with the TN-mode (twisted nematic mode) being the most commonly used driving method. However, in their use as a flat panel display, the TN display system has several limitations due to their relatively slow response time and narrow viewing angle, compared to the emissive type display systems such as electrolumilescence displays and plasma displays. Therefore, in order to broaden the acceptance of liquid crystal display devices and utilize the advantages thereof as illustrated hereinabove, developments of improved liquid crystal materials as alternative to the nematic phase type liquid crystal and improved driving methods are essential.
Recently, ferroelectric liquid crystals have received significant attention as an alternative to the nematic type liquid crystals. In the past decade and half, over 500 patents have been issued in the areas related to ferroelectric liquid crystals. The existence of ferroelectric liquid crystals was first observed in 1974 which was subsequently published in 1975 by R. B. Meyer, L. Liebert, L. Strzelecki, and P. Keller (see J. Physique Letters, 1975, 36, L-69). They reported that strong ferroelectricity could be observed from liquid crystals belonging to a chiral smectic C phase (Sc* phase). Prior to the discovery by Meyer, ferroelectricity was observed only in salt crystals. Meyer and his coworkers believed that ferroelectricity may be observed in optically active compounds with a tilted nematic liquid crystal phase. They synthesized a liquid crystal compound (S)-4-n-decyloxybenzylideneamino-2'-methylbutyl cinnamate (DOBAMBC) to prove their theory. Therefore, in theory, any optically active compound that exhibits tilted smectic phase can possess ferroelectricity. Currently, the chiral smectic C-phase (Sc*) compounds are receiving the most attention.
In 1980, N. A. Clark and S. T. Lagerwall proposed a liquid crystal display system wherein an optical switching phenomenon of a ferroelectric liquid crystal was utilized. (See Appl. Phys. Lett., 1980, 36, 899; see, also, U.S. Pat. No. 4,367,924, entitled "Chiral Smectic C or H Liquid Crystal Electro-Optical Device"). The discovery of Clark and Lagerwall opened the door for ferroelectric liquid crystals to be used in practical applications. One of the improvements advanced from their discovery is the use of surface stabilized ferroelectric liquid crystal device (SSFLCD) as a driving mechanism.
In order for the ferroelectric liquid crystals to be used as a display material, many conditions must be met. For example, the temperature range in which the Sc* phase is observed must be wide, and it must include room temperature. Furthermore, it must have high spontaneous polarization, Ps, in order to reduce the response time .tau.. The relationship between the spontaneous polarization Ps and the response time .tau. is shown below: EQU .tau.=.eta./Ps.times.E (1)
wherein .eta. is the viscosity of the ferroelectric liquid crystal, and E is the externally applied electrical voltage. From Eqn. (1), it is apparent that the response time of a ferroelectric liquid crystal can be reduced through appropriate molecular design to emphasize certain desired molecular characteristics. For example, a proper increase in the polarity of the ferroelectric molecule can increase the spontaneous polarization thereof and thus reduce the response time. However, Eqn. (1) also shows that viscosity is another important factor affecting the response time. Thus, in designing a ferroelectric molecule, we need to pay attention not only to the spontaneous polarization, but also to the viscosity of the molecule.
In practical applications, since a single molecule may not exhibit all the desired characteristics, a mixture of ferroelectric liquid crystals of various types, rather than a single type, are often used. The mixture can be prepared using doping or mixing methods (See W. Kuczynski, H. Stegemeyer, Chem. Phys. Lett., 1980, 70, 123; S. M. Kelly, A. Villiger, Displays, 1990, 41.) Under these circumstances, the compatibility between or among the various molecules becomes an important consideration.